Along with recent demands for smaller and lower profile electronic devices, finer semiconductor devices to be mounted onto these electronic devices have been increasingly demanded. Conventionally, a lithography method for manufacturing a semiconductor device has used a reduction projection exposure using ultraviolet (“UV”) light, but the minimum critical dimension transferable in the reduction projection exposure is in proportion to a wavelength of light used for transfer and in reverse proportion to a numerical aperture (“NA”) of a projection optical system. In order to transfer finer circuit patterns, a wavelength of used exposure light has been shortened from an i-line mercury lamp (with a wavelength of 365 nm) to KrF excimer laser (with a wavelength of 248 nm) and ArF excimer laser (with a wavelength of approximately 193 nm).
However, as the semiconductor device has rapidly become finer, the lithography using the UV light has a limited resolution. Accordingly, in order to efficiently print a very fine circuit pattern below 0.1 μm, a projection exposure apparatus has developed that uses EUV light having a wavelength between 10 and 15 nm, which is much smaller than that of the UV light.
The EUV light source uses, for example, a laser plasma light source. It uses YAG laser, etc. to irradiate a highly intensified pulse laser beam to a target material put in a vacuum chamber, thus generating high-temperature plasma for use as EUV light with a wavelength of about 13.5 nm emitted from this. The target material may use a metallic thin film, inert gas, and droplets, etc., and supplied to the vacuum chamber by gas jetting means and other means. The higher repetitive frequency of the pulse laser, e.g., repetitive frequency of typically several kHz, is preferable for the increased average intensity of the EUV light.
Japanese Patent Application Publications Nos. 5-217858, 8-236292, 11-40479, and U.S. Pat. No. 5,335,258 teach use of solid materials as the target material, while U.S. Pat. No. 5,459,771 teaches use of droplets as the target material.
Japanese Patent Application Publications Nos. 2003-43196 (corresponding to U.S. Patent Application Publication No. U.S. 2002/0162975 A1), 2000-110709, 2002-8891 and 2000-346817 teach use of a paraboloid-of-revolution mirror as a mirror for condensing the EUV light emitted from the generated plasma, while Japanese Patent Application Publications Nos. 2000-91209 (corresponding to U.S. Pat. No. 6,266,389) and 2001-332489 teach use of a ellipsoidal mirror.
The pulse laser beam with high intensity generates, when irradiating the target, flying particles called debris as well as the EUV light. The debris when adhering to an optical element causes pollution, damages and lowered reflectance, and thus debris removal means has conventionally been proposed to prevent the debris from reaching an optical element from the target. For example, a debris filter as one exemplary debris removal means is made of molybdenum, beryllium, zirconium, etc., and the transmittance to the EUV light is set between about 50% and 70%.
For easier prevention of the debris from entering the illumination optical system, it is preferable that the condenser mirror of the EUV light uses an ellipsoidal mirror that has one focal point where the plasma occurs, and another focal point for condensing light, as well as physically narrowing a path between the light source and the illumination system.
In principle, the EUV light is isotropically emitted from the plasma, and thus may be efficiently condensed when a cover angle of the condenser ellipsoidal mirror is made larger.
An illumination optical system that illuminates a target area using the EUV light is arranged below the emission point of the EUV light, and the conventional exposure apparatus arranges a laser light source such that an optical axis of the excitation laser accords with that of the EUV light incident upon the first mirror in the illumination optical system.
Due to vibrations and mechanical deformations, the excitation laser may go wide of the target entirely or partially. Due to causes other than a positional offset between the excitation laser and the target, the excitation laser may entirely or partially cross the emission point. When the excitation laser that crosses the emission point goes straight ahead and reaches the illumination optical system, it thermally deforms the first and subsequent mirrors or thermally destroys the multilayer on the mirror, lowering the resolution and hindering high-quality exposure. A repair and replacement of the mirror remarkably lowers the working efficiency of the apparatus since the illumination and projection optical systems are housed in a vacuum chamber. It is conceivable to considerably decrease a beam diameter of the laser light such that the excitation laser falls within the target even when it offsets slightly, but this undesirably decreases the power of the EUV light and lowers the throughput.
The above debris filter has transmittance between about 50% and 70% to the EUV light, but transmittance to a laser beam from YAG laser of about 100%. Therefore, the conventional debris filter has been insufficient to shield the laser beam that goes straight ahead beyond the EUV emission point.
Another conventional exposure apparatus arranges the laser light source such that an optical axis of the EUV light incident upon the first mirror in the illumination optical system does not accord with an optical axis of the excitation laser. However, this exposure apparatus restricts the cover angle of the condenser mirror so as to maintain the introduction space of the excitation laser to the target, and does not sufficiently high emission efficiency of the EUV light.
The prior art does not teach a large cover angle of the condenser ellipsoidal mirror, directions of the excitation laser incident onto a target and exiting the target, or interference between the excitation laser and the illumination system including the ellipsoidal mirror.